The tumor-necrosis factor (TNF)-related cytokines are mediators of host defense and immune regulation. Members of this family exist in membrane-anchored forms, acting locally through cell-to-cell contact, or as secreted proteins capable of diffusing to more distant targets. A parallel family of receptors signals the presence of these molecules leading to the initiation of cell death or cellular proliferation and differentiation in the target tissue. Presently, the TNF family of ligands and receptors has at least 11 recognized receptor-ligand pairs, including: TNF:TNF-R; LT-α:TNF-R; LT-α/β:LT-β-R; FasL:Fas; CD40L:CD40; CD30L:CD30; CD27L:CD27; OX40L:OX40 and 4-1BBL:4-1BB. The DNA sequences encoding these ligands have only about 25% to about 30% identity in even the most related cases, although the amino acid relatedness is about 50%.
The defining feature of this family of cytokine receptors is found in the cysteine rich extracellular domain initially revealed by the molecular cloning of two distinct TNF receptors. This family of genes encodes glycoproteins characteristic of Type I transmembrane proteins with an extracellular ligand binding domain, a single membrane spanning region and a cytoplasmic region involved in activating cellular functions. The cysteine-rich ligand binding region exhibits a tightly knit disulfide linked core domain, which, depending upon the particular family member, is repeated multiple times. Most receptors have four domains, although there may be as few as three, or as many as six.
Proteins in the TNF family of ligands are characterized by a short N-terminal stretch of normally short hydrophilic amino acids, often containing several lysine or arginine residues thought to serve as stop transfer sequences. Next follows a transmembrane region and an extracellular region of variable length, that separates the C-terminal receptor binding domain from the membrane. This region is sometimes referred to as the “stalk”. The C-terminal binding region comprises the bulk of the protein, and often, but not always, contains glycosylation sites. These genes lack the classic signal sequences characteristic of type I membrane proteins, type II membrane proteins with the C terminus lying outside the cell, and a short N-terminal domain residing in the cytoplasm. In some cases, e.g., TNF and LT-α, cleavage in the stalk region can occur early during protein processing and the ligand is then found primarily in secreted form. Most ligands, however, exist in a membrane form, mediating localized signaling.
The structure of these ligands has been well-defined by crystallographic analyses of TNF, LT-α, and CD40L. TNF and lymphotoxin-I (LT-I) are both structured into a sandwich of two anti-parallel β-pleated sheets with the “jelly roll” or Greek key topology. The rms deviation between the Cα and β residues is 0.61 C, suggesting a high degree of similarity in their molecular topography. A structural feature emerging from molecular studies of CD40L, TNF and LT-α is the propensity to assemble into oligomeric complexes. Intrinsic to the oligomeric structure is the formation of the receptor binding site at the junction between the neighboring subunits creating a multivalent ligand. The quaternary structures of TNF, CD40L and LTα have been shown to exist as trimers by analysis of their crystal structures. Many of the amino acids conserved between the different ligands are in stretches of the scaffold β-sheet. It is likely that the basic sandwich structure is preserved in all of these molecules, since portions of these scaffold sequences are conserved across the various family members. The quaternary structure may also be maintained since the subunit conformation is likely to remain similar.
TNF family members can best be described as master switches in the immune system controlling both cell survival and differentiation. Only TNF and LTα, are currently recognized as secreted cytokines contrasting with the other predominantly membrane anchored members of the TNF family. While a membrane form of TNF has been well-characterized and is likely to have unique biological roles, secreted TNF functions as a general alarm signaling to cells more distant from the site of the triggering event. Thus TNF secretion can amplify an event leading to the well-described changes in the vasculature lining and the inflammatory state of cells. In contrast, the membrane bound members of the family send signals though the TNF type receptors only to cells in direct contact. For example T cells provide CD40 mediated “help” only to those B cells brought into direct contact via cognate TCR interactions. Similar cell-cell contact limitations on the ability to induce cell death apply to the well-studied Fas system.
It appears that one can segregate the TNF ligands into three groups based on their ability to induce cell death. First, TNF, Fas ligand and TRAIL can efficiently induce cell death in many lines and their receptors mostly likely have good canonical death domains. Presumably the ligand to DR-3 (TRAMP/WSL-1) would also all into this category. Next there are those ligands which trigger a weaker death signal limited to few cell types and TWEAK, CD30 ligand and LTa1b2 are examples of this class. How this group can trigger cell death in the absence of a canonical death domain is an interesting question and suggests that a separate weaker death signaling mechanism exists. Lastly, there are those members that cannot efficiently deliver a death signal. Probably all groups can have antiproliferative effects on some cell types consequent to inducing cell differentiation e.g. CD40. Funakoshi et al. (1994).
The TNF family has grown dramatically in recent years to encompass at least 11 different signaling pathways involving regulation of the immune system. The widespread expression patterns of TWEAK and TRAIL indicate that there is still more functional variety to be uncovered in this family. This aspect has been especially highlighted recently in the discovery of two receptors that affect the ability of rous sacroma and herpes simplex virus to replicate as well as the historical observations that TNF has anti-viral activity and pox viruses encode for decoy TNF receptors. Brojatsch et al. (1996); Montgomery et al. (1996); Smith et al. (1994), 76 Cell 959-962; Vassalli et al. (1992), 10 Immunol. 411-452.
TNF is a mediator of septic shock and cachexia, and is involved in the regulation of hematopoietic cell development. It appears to play a major role as a mediator of inflammation and defense against bacterial, viral and parasitic infections as well as having antitumor activity. TNF is also involved in different autoimmune diseases. TNF may be produced by several types of cells, including macrophages, fibroblasts, T cells and natural killer cells. TNF binds to two different receptors, each acting through specific intracellular signaling molecules, thus resulting in different effects of TNF. TNF can exist either as a membrane bound form or as a soluble secreted cytokine.
LT-I shares many activities with TNF, i.e. binding to the TNF receptors, but unlike TNF, appears to be secreted primarily by activated T cells and some β-lymphoblastoid tumors. The heteromeric complex of LT-α, and LT-β is a membrane bound complex which binds to the LT-βreceptor. The LT system (LTs and LT-R) appears to be involved in the development of peripheral lymphoid organs since genetic disruption of LT-β leads to disorganization of T and B cells in the spleen and an absence of lymph nodes. The LT-β system is also involved in cell death of some adenocarcinoma cell lines.
Fas-L, another member of the TNF family, is expressed predominantly on activated T cells. It induces the death of cells bearing its receptor, including tumor cells and HIV-infected cells, by a mechanism known as programmed cell death or apoptosis. Furthermore, deficiencies in either Fas or Fas-L may lead to lymphoproliferative disorders, confirming the role of the Fas system in the regulation of immune responses. The Fas system is also involved in liver damage resulting from hepatitis chronic infection and in autoimmunity in HIV-infected patients. The Fas system is also involved in T-cell destruction in HIV patients. TRAIL, another member of this family, also seems to be involved in the death of a wide variety of transformed cell lines of diverse origin.
CD40-L, another member of the TNF family, is expressed on T cells and induces the regulation of CD40-bearing B cells. Furthermore, alterations in the CD40-L gene result in a disease known as X-linked hyper-IgM syndrome. The CD40 system is also involved in different autoimmune diseases and CD40-L is known to have antiviral properties. Although the CD40 system is involved in the rescue of apoptotic B cells, in non-immune cells it induces apoptosis. Many additional lymphocyte members of the TNF family are also involved in costimulation.
Generally, the members of the TNF family have fundamental regulatory roles in controlling the immune system and activating acute host defense systems. Given the current progress in manipulating members of the TNF family for therapeutic benefit, it is likely that members of this family may provide unique means to control disease. Some of the ligands of this family can directly induce the apoptotic death of many transformed cells e.g. LT, TNF, Fas ligand and TRAIL. Nagata (1997) 88 Cell 355-365. Fas and possibly TNF and CD30 receptor activation can induce cell death in nontransformed lymphocytes which may play an immunoregulatory function. Amakawa et al. (1996) 84 Cell 551-562; Nagata (1997) 88 Cell 355-365; Sytwu et al. (1996); Zheng et al. (1995) 377 Nature 348-351. In general, death is triggered following the aggregation of death domains which reside on the cytoplasmic side of the TNF receptors. The death domain orchestrates the assembly of various signal transduction components which result in the activation of the caspase cascade. Nagata (1997) 88 Cell 355-365. Some receptors lack canonical death domains, e.g. LTb receptor and CD30 (Browning et al. (1996); Lee et al. (1996)) yet can induce cell death, albeit more weakly. It is likely that these receptors function primarily to induce cell differentiation and the death is an aberrant consequence in some transformed cell lines, although this picture is unclear as studies on the CD30 null mouse suggest a death role in negative selection in the thymus. Amakawa et al. (1996) 84 Cell 551-562. Conversely, signaling through other pathways such as CD40 is required to maintain cell survival. Thus, there is a need to identify and characterize additional molecules which are members of the TNF family thereby providing additional means of controlling disease and manipulating the immune system.
Here we characterize the functional properties of a new ligand of the TNF cytokine family. The new ligand, termed BAFF (B cell activating factor belonging to the TNF family), appears to be expressed by T cells and dendritic cells for the purpose of B-cell co-stimulation and may therefore play an important role in the control of B cell function. In addition, we have generated transgenic mice overexpressing BAFF under the control of a liver-specific promoter. These mice have excessive numbers of mature B cells, spontaneous germinal center reactions, secrete autoantibodies, and have high plasma cell numbers in secondary lymphoid organs and Ig deposition in the kidney.